1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging, and more particularly, to a nuclear magnetic resonance imaging suitable for imaging physiological function information of the interior of the body to be examined at high precision.
2. Description of the Background Art
In recent years, many medical diagnostic systems using the nuclear magnetic resonance imaging (MRI) apparatus have been developed.
As well known, the nuclear magnetic resonance imaging is a method for imaging microscopic chemical and physical information of matters by utilizing the nuclear magnetic resonance phenomenon in which the energy of a radio frequency magnetic field rotating at a specific frequency can be resonantly absorbed by a group of nuclear spins having unique magnetic moments which are placed in a homogeneous static magnetic field.
In this nuclear magnetic resonance imaging, the images can be obtained in various contrasts such as the image in contrast emphasizing the longitudinal relaxation time T.sub.1 of the nuclear spins (T.sub.1 image), the image in contrast emphasizing the transverse relaxation time T.sub.2 of the nuclear spins (T.sub.2 image), the image in contrast emphasizing the density distribution of the nuclear spins (density image), and the image in contrast emphasizing the parameter T.sub.2 * which reflects both the transverse relaxation time T.sub.2 and the sudden phase change of the nuclear spins due to the microscopic magnetic field inhomogeneity within a voxel.
On the other hand, as described in S. Ogawa et al.: "Oxygenation-Sensitive Contrast in Magnetic Resonance Image of Rodent Brain at High Magnetic Fields", Magnetic Resonance in Medicine 14, pp. 68-78, 1990, it is known that, among the hemoglobin contained in blood of the living body, the oxyhemoglobin contained in abundance in the arterial blood is diamagnetic, while the deoxyhemoglobin mainly contained in the venous blood is paramagnetic. Then, as described in R. M. Weisskoff et al.: "MRI Susceptometry: Image-Based Measurement of Absolute Susceptibility of MR contrast Agents and Human Blood", Magnetic Resonance in Medicine 24, pp. 375-383, 1992, it is also known that the diamagnetic oxyhemoglobin does not disturb the local magnetic field very much (magnetic susceptibility difference of 0.02 ppm with respect to the living body tissues), but the paramagnetic deoxyhemoglobin has sufficiently large magnetic susceptibility difference with respect to the surrounding tissues (magnetic susceptibility difference of 0.15 ppm with respect to the living body tissues) to disturb the magnetic field so that the parameter T.sub.2 * is going to be shortened.
Also, as described in J. A. Detre, et al.: "Perfusion Imaging", Magnetic Resonance in Medicine 23, pp. 37-45, 1992, in some imaging schemes of the nuclear magnetic resonance imaging, when the amount or the speed of the local blood flow within the living body tissues, the relaxation time (such as T.sub.1) of the living body seemingly appears to have changed, and the image contrast can be changed.
By utilizing the above noted properties, it is possible to image the change of the blood flow or the change of the oxygen density in blood due to the physiological function such as the cell activity within the living body tissues including the activation of the visual area in the brain cortex caused by the light stimulation, as described for example in K. K. Kwong et al.: "Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation", Proc. Natl. Acad. Sci. USA, Vol. 89. pp. 5675-5679, June 1992. Conventionally, the imaging scheme used in this type of imaging has been the echo planar scheme using the pulse sequence as shown in FIG. 1 or the gradient echo scheme using the pulse sequence as shown in FIG. 2.
However, in these imaging schemes, the signal change (image contrast change) caused by the physiological function within the living body is quite minute. For this reason, conventionally, this minute signal change has been detected by calculating the difference or the correlation of the images before and after the occurrence of the physiological function phenomenon, as described in R. T. Constable, et al.: "Functional Brain Imagings at 1.5 T using Conventional Gradient Echo MR Imaging Techniques", Magnetic Resonance Imaging, Vol. 11, pp. 451-459, 1993. In addition, there has been an attempt to comprehend the physiological function quantitatively by calculating the change of the blood flow amount or the oxygen density in blood from the change of the contrast intensity or the phase in the images.
However, in such a conventional method, when the position displacement due to the body movement between two images occurs, it becomes impossible to detect such a minute change accurately. In fact, it is well known that the position and the size of the brain can change in synchronization with the heart beat, as described in B. P. Poncelet, et al.: "Brain Parenchyma Motion: Measurement with Cine Echo-Planar MR imaging", Radiology, Vol. 185, pp. 645-651, December 1992. Thus, in the conventional method, because of the influence of the body movement due to the breathing or the heart beat, it has been impossible to accurately detect the signal change (image contrast change) caused by the physiological function such as the cell activity in the living body.
On the other hand, it is also well known that the image distortion can be caused in the nuclear magnetic resonance imaging when the static magnetic field distribution is inhomogeneous, and this image distortion becomes particularly noticeable in the imaging scheme for the T.sub.2 * image which is used in detecting the physiological function phenomenon such as the cell activity in the living body. However, when such an image distortion is present, it is impossible to accurately detect the position of the physiological function change such as the cell activity in the living body.
Moreover, in a case of calculating an average image from a plurality of images obtained by the repeated imaging operations in order to improve the signal to noise ratio of the image, or carrying out the processing among a plurality of images in order to detect the physiological function phenomenon such as the cell activity in the living body, when the signal strength or the imaged portion changes depending on the imaging conditions or the system states, it is impossible to accurately detect the physiological function change such as the cell activity in the living body by the processing among the images.